If Juve prefer to include Matri in a cash+player deal that's solely because Matri can lower down the cash offer much more than Quag.
Realistically Matri's price is some 13-14m euro while Quag's is 7-8m. But if we put them on the market noone will pay the money. The Italian teams don't have the cash while the foreign teams won't find these two attractive and worthy to pay the money (well, maybe the Russians will, but Quag and Matri won't go there). The only way to get close to their value is if we include them in a player+cash deal. That way we can offer 14+Matri for Jovetic, which would equal some 27-28m eur. Quag+cash would require ~20m eur cash. That's why we would prefer to include Matri in a deal for Jovetic. In any case, neither of Matri/Quag seems to be in Conte's plans atm and both will be 4th choice strikers if we end up buying Jojo.